An ion source is a device that emits ions from a plasma and preferably forms ion beams. The ion beams can be used for various technological applications such as cleaning, activation, polishing, etching, thin film deposition, or ion thruster.
The energetic beams of ions can be emitted by applying electrostatic and electromagnetic acceleration methods. An example of an electrostatic acceleration ion source, the so-called Kaufman ion source, also known as a Kaufman ion thruster or electron bombardment ion source, is described in U.S. Pat. No. 4,481,062 issued to H. R. Kaufman in 1984, which is incorporated herein by reference. A disadvantage of such an ion source is that it requires the use of ion accelerating grids and has a low intensity ion beam.
A hall type ion source, in which the electromagnetic acceleration is used, was introduced by Kaufman in 1989 in U.S. Pat. No. 4,862,032. Some modified sources are also described in U.S. Pat. No. 6,608,431 B1 and U.S. Pat. No. 6,750,600 B2 by Kaufman, and in U.S. Pat. No. 6,645,301 B2 by Sainty. These are gridless ion sources with a discharge chamber determined by the conical shape of a hollow anode and therefore also called end-hall ion sources with a circular discharge region and only an outside open boundary. The ion beams are generated and accelerated continuously over the circular cross section in crossed electric and magnetic fields, whose density is several hundred times higher than that offered by electrostatic acceleration due to the presence of electrons to avoid the disruptive mutual repulsion of positive ions. These publications are also incorporated herein by reference.
Electromagnetic acceleration ion sources, which are referred to as gridless ion sources, include two common types, which are both also called hall ion sources. One is the aforementioned end hall ion source and the other is a closed drift ion source with an annular discharge channel. Both of these ion sources utilize the hall effect which plays a major role in plasma discharge and ion acceleration.
Closed drift ion sources have been known since Russian ion thrusters for satellite propulsion were reported in 1960's. There are two main types of closed drift ion source distinguished by the length and materials of the discharge channel, namely the magnetic layer ion source and the anode layer ion source. The first one has a long discharge channel, which is usually made of dielectric materials, whereas the second one has a short discharge channel and its wall is made of conductive materials. Both of them have very similar characteristics and performance, but with some non-fundamental differences. They usually have radial magnetic field shape as shown in U.S. Pat. No. 5,359,258 by Arkhipov and U.S. Pat. No. 5,763,989 by Kaufman, and as described by Zhurin, in Plasma Source Science and Technolog, Vol. 8 (1999), beginning on page R1. These publications are also incorporated herein by reference.
Anode layer ion sources produce and accelerate ions from the thin and intense plasma ignited ahead of anode electrode by plasma discharge in the strong crossed electric and magnetic fields. Besides annular ionization channel in the ion thruster, a different closed drift channel may also be used in the anode layer ion source. The linear type may use race-track channel to form a continuous and closed ionization section as shown in Advanced Energy Industries, Inc. Industrial ion sources and their application for DLC coating, mentioned in U.S. Pat. No. 20050247885 A1 by Madocks and in U.S. Pat. No. 6,984,942 B2 by Seigfried et al. These publications are incorporated herein for reference.
In a conventional end hall ion source, the circular geometry is not well suited for scaling to a large size because of the central magnetic pole and central gas feed-through configuration. Furthermore, the annular closed drift channel design is not suited for scaling to a small size because the small size for annular design has a great deal of surface area relative to the volume. Therefore, large size circular ion beams, like that of the Kaufman grid ion source, cannot be achieved conventionally with these two types of ion sources.
Some cylindrical geometry hall sources, which comprise characteristics of end hall and closed electron drift, were described in U.S. Pat. No. 20020014845 A1 issued to Raitses et al, in 2002 for small power ion thrusters and in U.S. Pat. No. 20050237000 A1 issued to Zhurin in 2005 for high efficient ion sources. These publications are also incorporated herein by reference.